OK, I can see it in Sonar, but I can't find it in the Sonar Theme Editor. The only icon that seems to fit is the Add Lane icon in the Track View / Track Pane section, or the Add Module icon from the Pro Channel section.
If this is indeed either one of them, then it's a shared icon and modifying it implies that it has to fit with the design you choose in those different parts of Sonar.